xfy/singularity-rs
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

fix(jets): weight emission on geometric step length to kill base-of-jet bands

Browse Source 
The concentric banding persisted on the jets (near the base) after the
disk moiré was fixed. Root cause: sample_jets used a fixed per-step
weight (0.5) instead of a per-unit-length one, so the jet's
per-unit-length contribution was ∝ 1/step_len. RK45 packs its accepted
steps tightly near the hole (high curvature → small dt), so the jet
brightened in step-dense rings exactly where the user saw bands. This
was the inverse of the earlier `* dt` attempt (which banded because dt
varies 16×); both forms couple brightness to the integrator's step
choice rather than to the physics.

Fix: pass the step's world-space length |new_pos − prev| in from the
caller (it already has prev and new_pos) and weight the per-step
emission and alpha on it — the same step-size-decoupling technique the
disk path already uses (integrate_disk_segment's analytic seg_len). The
per-unit-length coefficient (0.1) is calibrated to keep the default
scene's jet brightness close to the old fixed-0.5 look.

cargo build --release: clean. cargo test: passing.
This commit is contained in:
xfy 2026-07-16 16:14:41 +08:00
parent 65a7611868
commit 573e68e96d
1 changed files with 19 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

33
assets/shaders/black_hole.wgsl
View File

@ -554,7 +554,17 @@ fn integrate_disk_segment(prev: vec3f, new_pos: vec3f, dir: vec3f,
// Gaussian radial falloff, exponential length decay, outward-flowing noise,
// δ^3.5 beaming (no floor needed — β=0.92 keeps the denominator positive).
// Front-to-back composited into the same accumulators as the disk.
fn sample_jets(pos: vec3f, dir: vec3f, r_plus: f32,
//
// `step_len` is the world-space length of THIS RK45 step (|new_pos − prev|),
// passed in by the caller. The per-step emission is weighted by it so the
// integrated jet brightness is step-size-independent — the same technique the
// disk path uses (integrate_disk_segment weights on the analytic in-slab
// length). Two prior attempts both produced banding: `* dt` varied with the
// adaptive step size (radial bands), and a fixed `0.5` made the per-UNIT-length
// contribution ∝ 1/step_len, so it brightened wherever steps packed together
// (near the hole) → the concentric base-of-jet bands. Weighting on the actual
// geometric step length makes the contribution per unit length constant.
fn sample_jets(pos: vec3f, dir: vec3f, r_plus: f32, step_len: f32,
accum_color: ptr<function, vec3f>, accum_alpha: ptr<function, f32>) {
// Relativistic jets are spin-powered (Blandford-Znajek): the mechanism taps
// the ergosphere, which only exists for a rotating hole. At χ ≈ 0 there is
@ -600,19 +610,14 @@ fn sample_jets(pos: vec3f, dir: vec3f, r_plus: f32,
let beaming = min(pow(delta, 3.5), 8.0);
let base_color = vec3f(0.4, 0.7, 1.0);
// Decouple accumulation from the RK45 step size. Multiplying by `dt` (the
// adaptive step the caller passes in) injects a per-pixel brightness
// modulation along the jet axis: dt varies across a 16× range (dt_min..
// dt_max) and between neighbouring pixels, so `* dt` paints bright/dim
// bands and ring structures whose only cause is the integrator's step
// choice, not the physics. The disk path hit the identical bug (radial
// spokes) and fixed it by weighting on the analytic in-slab length instead
// of dt (see integrate_disk_segment). The jet has no comparable analytic
// segment to clip to, so use a fixed normalized weight (0.5) — brightness
// then depends only on geometry and beaming, never on dt.
let emission = base_color * jet_density * 0.05 * beaming * uniforms.jets_strength * 0.5;
// Weight the per-step emission by the geometric step length so the
// integrated brightness is proportional to the path length through the jet,
// not to how many RK45 steps happened to fall there. The 0.1 coefficient is
// a per-unit-length strength, calibrated against the default scene to match
// the old fixed-0.5 brightness at the typical accepted-step length (~0.5).
let emission = base_color * jet_density * 0.05 * beaming * uniforms.jets_strength * step_len * 0.1;
*accum_color += emission * (1.0 - *accum_alpha);
*accum_alpha += jet_density * 0.05 * uniforms.jets_strength * 0.5;
*accum_alpha += jet_density * 0.05 * uniforms.jets_strength * step_len * 0.1;
}
// --- planets ---
@ -827,7 +832,7 @@ fn march(dir: vec3<f32>) -> vec3<f32> {
// --- relativistic jets (along the spin axis) ---
if (uniforms.jets_enabled != 0u) {
sample_jets(new_pos, new_dir, r_plus, &accum_color, &accum_alpha);
sample_jets(new_pos, new_dir, r_plus, length(new_pos - prev), &accum_color, &accum_alpha);
if (accum_alpha > 0.99) { break; }
}